The Galactic Center Interstellar Medium: from ISO to FIRST

The Infrared Space Observatory (ISO) has shown the complexity of the Galactic center (GC) Interstellar medium (ISM) detecting, not only large column densities of warm molecular gas (H2), but the emission of neutral atoms and ions of low ionization potential (CII, OI, SiII,...) that should arise in shocked or photon-dominated regions (PDRs). In addition, ISO has also detected emission from ions like SIII, NeII, ArII, or NII (in some clouds we have even detected NeIII and OIII) that should arise from HII regions that were previously unsuspected due to the non-detection of Hydrogen recombination lines. Here we review some ISO results on the large scale study of the GC ISM and in particular, on the heating mechanisms of the clouds. Although, shocks should play an important role on the physics and chemistry of the GC ISM, ISO shows that the effect of radiation on the heating of the gas cannot be ruled out with the simple argument that the dust temperature is lower than that of the gas.